In the field of microwave and millimeter detection, it is a common practice to provide a predetermined bias voltage on a semiconductor diode in order to set the DC operating point on a nonlinear region of the diode's current-voltage (I-V) characteristic in order to provide for maximum detection efficiency. Typically, this DC bias voltage is on the order of 0.7 volts where the diode is highly conductive in a range on one side of the DC bias voltage point and rather lighty conductive in a range on the other side of this DC bias voltage.
There are, however, several significant disadvantages in the requirement for a DC bias voltage on the diode detector. First, the requirement per se of DC bias circuitry between a power supply and the detector diode adds cost and complexity to the detector arrangement. Secondly, there is a certain amount of noise associated with the DC bias voltage, and this noise degrades the sensitivity and decreases the dynamic range of the detection. Thirdly, the bias voltage is thermally sensitive and will therefore frequently cause the DC bias operating point on the diode's I-V characteristic to be shifted in response to changes in ambient temperature at the DC bias source.
Similarly, when employing mixer diodes, it has been a common practice to provide a DC bias voltage across the mixer diode or diodes in order to establish a desired operating point on the I-V characteristic of these diodes. One such mixer diode structure is disclosed for example by Malik in U.S. Pat. No. 4,410,902. However, an additional problem in the Malik mixer structure arises from the fact that there will be some extraneous and undesirable doping in the body of the Malik structure from impuritis in the substrate moving upwardly into the epitaxial layers thereon. This doping results in an unevenly distributed and extraneous impurity profile across these layers, and this profile in turn produces dissimilar and asymmetrical I-V curves in the first and third quadrants of the device's composite I-V characteristic. Such asymmetrical I-V characteristics ultimately result in the generation of unwanted odd harmonic signals of the fundamental mixing frequency.
For a further discussion of this problem of extraneous doping in mixer structures of the type disclosed by Malik in U.S. Pat. No. 4,410,902, reference may be made to an article by S. C. Palmateer, et al., entitled "A study of substrate effects on planar doped structures in gallium arsenide grown by molecular beam epitaxy", Institute Physics Conference, Ser. No. 65: Chapter 3, presented at the International Symposium of Gallium Arsenide and Related Compounds, Albuquerque, 1982, at page 149 et seq.
In the field of diode detection there has been at least one attempt to provide a detector diode which operates with zero bias. Such an attempt is evidenced for example in U.S. Pat. No. 3,968,272 issued to Anand. However, the Anand device relies upon the reaction of a semiconductor surface with certain metals in a controlled manner. It is well known that such semiconductor surface chemistry is difficult to control and this fact will in turn affect device yields and repeatability of results. Additionally, using the Anand process stable barrier heights have only been demonstrated in silicon which has a lower electron velocity than gallium arsenide, and thus operates at slower speeds than GaAs, a fact which further contributes to the slower speeds of silicon devices.